Architecture for TDMA medical telemetry system

ABSTRACT

A medical telemetry system is provided for collecting the real-time physiologic data of patients (including ambulatory patients) of a medical facility, and for transferring the data via RF to a real-time data distribution network for monitoring and display. The system includes battery-powered remote telemeters which attach to respective patients, and which collect and transmit (in data packets) the physiologic data of the patients. The remote telemeters communicate bi-directionally with a number of ceiling-mounted RF transceivers, referred to as &#34;VCELLs,&#34; using a wireless TDMA protocol. The VCELLs, which are hardwire-connected to a LAN, forward the data packets received from the telemeters to patient monitoring stations on the LAN. The VCELLs are distributed throughout the medical facility such that different VCELLs provide coverage for different patient areas. As part of the wireless TDMA protocol, the remote telemeters continuously assess the quality of the RF links offered by different nearby VCELLs (by scanning the frequencies on which different VCELLs operate), and connect to those VCELLs which offer the best link conditions. To provide a high degree of protection against multi-path interference, each remote telemeter maintains connections with two different VCELLs at-a-time, and transmits all data packets (on different frequencies and during different timeslots) to both VCELLs; the system thereby provides space, time and frequency diversity on wireless data packet transfers from the telemeters. The telemeters and VCELLs also implement a patient location protocol for enabling the monitoring of the locations of individual patients. The architecture can accommodate a large number of patients (e.g., 500 or more) while operating within the transmission power limits of the VHF medical telemetry band.

PRIORITY CLAIM

This application claims the benefit of U.S. Provisional Appl. No.60/006,600 titled TWO-WAY TDMA TELEMETRY SYSTEM, filed Nov. 13, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to digital wireless communications systemsof the type which employ portable, battery-powered communicationsdevices, such as remote telemeters worn by ambulatory hospital patientsfor monitoring purposes. More particularly, the present inventionrelates to a network architecture, and an associated TDMA (time divisionmultiple access) communications protocol, for facilitating the efficientand reliable exchange of information between portable wireless devicesand centralized monitoring stations.

2. Description of the Related Art

Medical telemetry systems that allow the physiologic data of multiple,remotely-located patients to be monitored from a central location areknown in the art. These systems typically comprise remote telemetersthat remotely collect the physiologic data of respective patients andtransmit the data over a wireless link to a centralized monitoringstation. This physiologic data may include, for example, real-timeelectrocardiograph (ECG) waveforms, CO₂ levels, and temperaturereadings. From the centralized monitoring station, a clinician canvisually monitor the physiologic status, in real time, of many differentpatients. The central station may also run automated monitoring softwarefor alerting the clinician whenever a predetermined physiologic eventoccurs, such as a cardiac arrythmia condition.

Remote telemeters of medical telemetry systems are generally of twotypes: instrument remote telemeters and ambulatory remote telemeters. Anambulatory remote telemeter is a portable, battery-powered device whichpermits the patient to be monitored while the patient is ambulatory. Theambulatory telemeter attaches to the patient by a strap or otherattachment device, and receives the patient's physiologic data via ECGleads (and/or other types of sensor leads) which attach to the patient'sbody. The physiologic data is continuously transmitted to the centralmonitoring station by the telemeter's RF (radio frequency) transmitterto permit real-time monitoring. (A design of a remote transceiver whichmay be used in a two-way, ambulatory telemeter is described in theabove-referenced provisional application.) Instrument remote telemetersoperate in a similar manner, but receive the patient's physiologic datafrom a bedside monitor (or other instrument) over a hardwired link, suchas an RS-232 connection. Instrument remote telemeters that transfer thephysiologic data to the central station over a hardwired connection arealso common.

SUMMARY OF THE INVENTION

One problem that is commonly encountered in the field of medicaltelemetry involves signal loss caused by multi-path interference.Multi-path interference is a well-known phenomenon which occurs when asignal takes two or more paths (as the result of signal reflections)from the transmitter to the receiver such that the multi-path componentsdestructively interfere with each other at the receiver's antenna. Toreduce the effects of multi-path interference, some telemetry equipmentmanufactures have included multiple antenna/receiver pairs on eachremote telemeter. With this technique, known as spacial diversity, whenone of the antennas experiences multipath fading, the other antenna (andthe corresponding receiver) is used to receive the signal. One problemwith this method is that it adds to the cost, size and complexity of theremote telemeter. In addition, in at least some implementations, a lossof data may occur when a "switch-over" is performed from oneantenna/receiver pair to the other.

Another problem that has been encountered in the field of medicaltelemetry relates to the ability to monitor a large number of patientsover a coverage area that extends to all patient areas of the hospital.A common solution to this problem involves installing a large number ofantennas (e.g., 200 or more) throughout the hospital (with differentantennas positioned in different patient areas), and interconnecting theantennas using signal combiners to form a single, distributed antennasystem. One problem with this "distributed antenna system" approach isthat each antenna and its associated preamplifier (or preamplifiers)contributes to the noise floor of the antenna system, and therebyincreases the minimum transmit power at which the transmittingcomponents of the system can operate. (The reasons for this noise floordegradation are discussed below.) Consequently, unless the transmissionpower of the system's transmitters is increased, a practical limitationis imposed on the number of antennas that can be included in the system,and on the coverage area provided by the system.

Although the noise floor degradation problem can potentially be overcomeby increasing the transmission power of the telemetry equipment, thereare at least two problems associated with increasing the transmit power.The first problem is that under existing Federal CommunicationsCommission (FCC) regulations, medical telemetry equipment is onlypermitted to operate within certain frequency bands, and must operatewithin certain prescribed power limits within these bands. Under FCCPart 15.241, for example, which governs the protected VHF (174-216 MHz)medical telemetry band (a band which is generally restricted to VHFtelevision and medical telemetry), telemetry devices are not permittedto transmit at a signal level which exceeds 1500 microvolts/meter at 3meters. To operate at power levels which exceed this maximum, frequencybands which offer less protection against interference must be used. Thesecond problem is that increasing the transmit power of an ambulatorytelemeter will normally produce a corresponding reduction in thetelemeter's battery life.

Another problem with distributed antenna systems is that they aretypically highly vulnerable to isolated sources of electromagneticinterference ("EMI"). Specifically, because the signals received by allof the antennas are combined using RF signal combiners, a single sourceof interference (such as a cellular phone or a faulty preamplifier) ator near one of the antennas can introduce an intolerable level of noiseinto the system, potentially preventing the monitoring of all patients.One consequence of this problem is that antennas generally cannot bepositioned near known intermittent sources of EMI such as X-raymachines, CAT (computerized axial tomography) scanners, and fluoroscopymachines, preventing patient monitoring in corresponding diagnosticareas.

In light of these and other problems with existing medical telemetrysystems, the present invention seeks to achieve a number ofperformance-related objectives. One such objective is to provide anarchitecture in which the coverage area and patient capacity can beincreased without degrading the noise floor. This would allow thetelemetry system to be expanded in size and capacity without the need toincrease the transmit power of the battery-powered remote telemeters,and without the need to operate outside the protected VHF medicaltelemetry band. A related objective is to provide an architecture whichis highly scalable, so that the capacity and coverage area of the systemcan easily be expanded through time.

Another goal of the invention is to provide extensive protection againstsignal drop-outs caused by multi-path interference. The presentinvention seeks to achieve this objective without the need for multipleantennas or receivers on the telemeters, and without the loss orinterruption of physiologic data commonly caused by antenna/receiverswitch-overs. A related goal is to provide a high degree of protectionagainst isolated sources of EMI, and to allow patients to be remotelymonitored while near known intermittent sources of interference.

Another goal of the invention is to provide an architecture in which alarge number of patients (e.g., 500 to 800 or more) can be monitoredusing a relatively narrow range of RF frequencies (such as theequivalent of one or two VHF television channels). This would allow theRF communications components of the system to be optimized fornarrow-band operation, which would in-turn provide a performanceadvantage over wide-band systems.

In accordance with these and other objectives, a medical telemetrysystem is provided which includes multiple remote telemeters (which mayinclude both ambulatory and instrument telemeters) which transmit thereal-time physiologic data of respective patients via RF to multipleceiling-mounted transceivers, referred to as "VCELLs." The VCELLs arehardwire-connected to a real-time data distribution network whichincludes at least one centralized monitoring station. (In a preferredimplementation, each group of 16 VCELLs is connected via twisted pairlines to a respective "concentrator PC," and the concentrator PCs andmonitoring stations are interconnected as part of a hospital local areanetwork.)

The VCELLs are distributed throughout the hospital such that differentVCELLs provide coverage for different patient areas, and are spaced suchthat the coverage zones provided by adjacent VCELLs overlap with oneanother. Different VCELLs within the same general area communicate withthe remote telemeters on different respective RF frequencies (i.e.,frequency channels), so that a remote telemeter can selectivelycommunicate with a given VCELL by selecting that VCELL's frequency. Asdescribed below, however, VCELL frequencies are reused by VCELLs thatare spaced sufficiently apart from one another to avoid interference,allowing the system to be implemented as a narrow-band system which usesa relatively small number of frequencies (e.g., 10) to provide coveragefor an entire hospital facility.

In a preferred embodiment, the remote telemeters communicate with theVCELLs using a wireless time division multiple access (TDMA) protocol inwhich each VCELL can concurrently receive the real-time physiologic dataof up to six remote telemeters (corresponding to six patients). As partof this protocol, the remote telemeters implement a VCELL "switch-over"protocol in which the telemeters establish wireless connections withdifferent VCELLs based on periodic assessments (made by the telemeters)of the wireless links offered by the different VCELLs. Thus, as apatient moves throughout the hospital, the patient's remote telemetermay connect to (and disconnect from) many different VCELLs.

In operation, the remote telemeters send data packets (during assignedtimeslots) to the respective VCELLs with which the telemeters haveestablished wireless connections. (As described below, each remotetelemeter preferably remains connected to two different VCELLs at-a-timeto provide extensive protection against multi-path interference.) Thesedata packets include the real-time physiologic data of respectivepatients, and include ID codes which identify the remote telemeters. TheVCELLs in-turn forward the data packets to the real-time datadistribution network to permit the real-time monitoring of the patientsof the system.

To provide protection against multi-path interference and other causesof data loss, each remote telemeter maintains wireless connections withtwo different VCELLs at-a-time, and transmits each data packet to bothof the VCELLs. These duplicate packet transmissions to the two differentVCELLs take place on different frequencies during different TDMAtimeslots. The two VCELLs forward the data packets to a centralized node(which may be a monitoring station or a concentrator PC in the preferredembodiment), which performs error correction by selecting between thecorresponding packets based on error detection codes contained withinthe packets. Thus, the patient's physiologic data is sent from theremote telemeter to the centralized node over two separate data paths.Because the two VCELLs are spaced apart, and because the duplicatepackets are transferred to the VCELLs on separate frequencies atdifferent times, the packet transfers benefit from the protectionoffered by spacial diversity, frequency diversity and time diversity.

The architecture of the above-described medical telemetry systemprovides numerous advantages over prior art systems. One such advantageis that the system can be expanded in patient capacity and coveragearea, by the addition of VCELLs, without increasing the noise floor ofthe system beyond the natural thermal noise floor. (This is because thedata signals received by the VCELLs are multiplexed digitally atbaseband, rather than being combined by RF analog signal combiners.)Thus, unlike distributed antenna telemetry systems, the noise floor doesnot impose an upper limit on the size of the system. Moreover, thearchitecture can accommodate a large number of patients (e.g., 500 to800 or more) using a low maximum transmission power, such as the maximumtransmit power permitted by the FCC for operation within the VHF medicaltelemetry band.

Another advantage is that the architecture is highly immune to isolatedsources of EMI. A source of EMI (such as a cellular phone), for example,will typically contaminate the signals received by no more than one ortwo nearby VCELLs, as opposed to introducing noise into the entiresystem. (Because the remote telemeters connect to two VCELLs at-a-time,and automatically switch to different VCELLs when bad link conditionsare detected, the contamination of one or two VCELLs will typicallyresult in little or no loss of telemetry data.) One benefit of thisimmunity is that VCELLs can be installed within X-ray rooms and otherradiological diagnostic rooms which contain intermittent sources of EMI,allowing patients to be monitored in such areas.

Another advantage of the architecture is that it permits the reuse of RFfrequencies by VCELLs that are sufficiently spaced apart (by about 500feet in a VHF implementation) to avoid interference with each other. Byextending this concept, the present invention provides coverage for theentire facility using a relatively small number of frequencies whichfall within a relatively narrow frequency band. In a preferred VHFimplementation, for example, it is estimated that a typical hospital canbe covered using only 10 to 12 VCELL frequencies which fall within afrequency band that is equal in width to about two adjacent VHFtelevision channels. This characteristic of the architectureadvantageously allows the telemeter transceivers to be optimized(through the appropriate selection of transceiver components) for arelatively narrow band of frequencies, which in-turn improvesperformance.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention are described below withreference to the drawings of a preferred embodiment, which is intendedto illustrate and not to limit the invention:

FIG. 1 is an architectural drawing of the hardware components of amedical telemetry system in accordance with the present invention.

FIG. 2 illustrates the attachment of an ambulatory remote telemeter to apatient of the system.

FIG. 3 illustrates the basic hardware components of the concentrator PCsand the ceiling-mounted transceivers (VCELLs) of FIG. 1.

FIG. 4 illustrates the basic hardware components of the ambulatoryremote telemeters of FIG. 1.

FIG. 5A is a generalized circuit diagram of a transceiver which may beused in the VCELLs and remote telemeters of the telemetry system.

FIG. 5B illustrates the output of the phase-locked loop (PLL) chip ofFIG. 5A during the locking of the transmit frequency of a remotetelemeter.

FIG. 6 illustrates an increase in dynamic range achieved by the presentsystem over prior art telemetry systems.

FIG. 7 illustrates how VCELLs operating on different frequencies may bearranged within a hospital hallway in accordance with the invention.

FIG. 8 illustrates a TDMA frame of a wireless TDMA protocol used for thetransfer of information between the remote telemeters and the VCELLs ofthe system.

FIG. 9 illustrates the basic timeslot status information stored by eachVCELL as part of the wireless TDMA protocol.

FIG. 10 is a flowchart of a protocol followed by each VCELL as part ofthe wireless TDMA protocol.

FIG. 11 illustrates the basic VCELL status information stored by eachremote telemeter as part of the wireless TDMA protocol.

FIG. 12 is a flowchart of a protocol followed by each remote telemeteras part of the wireless TDMA protocol.

FIG. 13 is a flow chart of a protocol followed by the concentrators PCsfor processing packets received from the VCELLs.

In the drawings, the left-most digit (or digits) of each referencenumber indicates the figure in which the item first appears. Forexample, an element with the reference number 310 first appears in FIG.3, and an element with reference number 1100 first appears in FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To facilitate a complete understanding of the invention, the descriptionof the preferred embodiment is arranged within the following sectionsand subsections:

1. OVERVIEW

(i) GENERAL OPERATION

(ii) HARDWARE COMPONENTS

(iii) PATIENT CAPACITY AND DATA THROUGHPUT

(iv) NOISE FLOOR IMPROVEMENT

(v) PROTECTION AGAINST ISOLATED EMI SOURCES

(vi) VCELL SPACING AND FREQUENCY REUSE

2. COMMUNICATIONS BETWEEN REMOTE TELEMETERS AND VCELLS

(i) OVERALL WIRELESS TDMA PROTOCOL

(ii) VCELL PROTOCOL

(iii) REMOTE TELEMETER PROTOCOL

3. COMMUNICATIONS BETWEEN VCELLS AND CONCENTRATORS

(i) PROCESSING OF TELEMETER COMMANDS

4. DATA TRANSFERS OVER LAN

5. VCELL LOAD MONITORING

6. TRANSCEIVER CIRCUIT AND OPERATION

7. CONCLUSION

1. Overview (FIGS. 1-7)

FIG. 1 illustrates the general architecture of a two-way medicaltelemetry system which operates in accordance with the presentinvention. The system, referred to herein as the "VCELL system,"includes a number of wireless remote telemeters 102A, 102B whichcollect, packetize and transmit the physiologic data of respectivehospital patients. (As used herein, the term "wireless" means that datais transferred to and/or from the device over a wireless medium.) Theremote telemeters 102 may include both patient-worn (ambulatory) remotetelemeters 102A which connect directly to the patient (as generallyillustrated in FIG. 2), and instrument remote telemeters 102B whichconnect to a bedside or other patient monitor 104. The physiologic datatransmitted by the remote telemeters may include, for example, real-timeECG signals, blood pressure readings, CO₂ levels, and temperaturereadings. The remote telemeters 102 may additionally sense and transmitvarious types of non-physiologic data, such as battery-level statusdata, ECG loose-lead status data, and patient location data. (The term"patient data" is used herein to refer collectively to the physiologicand non-physiologic data captured by the remote telemeters 102.)

The remote telemeters 102 communicate bi-directionally with a number ofceiling-mounted radio transceivers 106, referred to as "VCELLS," using atime division multiple access (TDMA) protocol. In one mode of operation,each VCELL 106 can communicate with up to six remote telemeters 102at-a-time at a rate of 10 kilobaud (Kbaud) per telemeter. The VCELLs 106are spaced apart from one another (typically by about 50 to 75 feet,depending upon expected patient density) throughout the hospital toprovide a "cell-like" coverage area which consists of overlapping zonesof coverage.

Different VCELLs 106 of the system operate (i.e., transmit and receivedata) on a different RF frequency channels ("frequencies") within theVHF medical telemetry band (174-216 MHz). However, VCELLs that aresufficiently spaced apart to avoid interference with one another mayoperate on like frequencies, as described below. The VCELLs 106 andtelemeters 102 of the preferred embodiment operate in compliance withthe spectrum utilization and transmission power limitations of FCC Part15.241. Although the system preferably operates within the VHF medicaltelemetry band, other suitable frequency bands may be used. In addition,although the system uses frequency division multiplexing to separate thedata transmissions to and from different VCELLs 106, other channelseparation techniques can be used.

Although the remote telemeters 102 and VCELLs 106 shown in FIG. 1 are ofthe type which communicate by radio frequency (RF), the system may alsoinclude "hardwired" remote telemeters and VCELLs which communicate overhardwire connections. For purposes of this description, however, it maybe assumed that the terms "remote telemeter" and "VCELL" refer to RFdevices, except where indicated otherwise.

With further reference to FIG. 1, the VCELLs 106 are connected byconventional shielded twisted pair lines 110 to concentrator PCs 112("concentrators"). In the preferred embodiment, each concentrator 112can accommodate up to sixteen VCELLs 106. In a typical hospitalinstallation, one concentrator 112 will service a single floor of thehospital. The concentrators 112 provide connectivity between the VCELLs106 an a hospital local area network (LAN) 116. The LAN 116 serves as areal-time data distribution system for distributing the physiologic dataof the patients with a known latency. The LAN 116 includes a 100Mbit/second backbone 118 which is based on the 100BaseTx (Ethernet)protocol. (The term "backbone" refers generally to the transmissionmedium and the networking cards of the LAN.) Alternative LAN protocolswhich could be used include ATM (Asynchronous Transfer Mode) and FDDI(Fiber Distributed Data Interface) and others.

The LAN 116 includes multiple monitoring stations 120 for allowinghospital personnel to remotely view and otherwise monitor the real-timephysiologic data of the patients of the system. Each monitoring station120 is preferably in the form of a standard 486 or Pentium based PC(personal computer) which runs conventional patient monitoring software,such as the VCOM (MPC 1100) patient monitoring software packageavailable from VitalCom Incorporated. The patient monitoring softwarecan also be loaded onto the concentrator PCs 112 so that theconcentrators double as monitoring stations. The LAN 116 may alsoinclude one or more gateway computers 124 for connecting the LAN 116 toother networks, such as the Internet, to permit the exchange of patientinformation with other medical facilities and patient sites.

As will be apparent, the architecture illustrated in FIG. 1 provides fora high degree of scalability. The system can initially be installed, forexample, as a single concentrator PC 112 which serves as the solemonitoring station for a set of 16 (or fewer) VCELLs, which may includeboth RF and hardwired VCELLs. With the addition of a LAN, new VCELLs 106and concentrators 112 can be added to increase the patient capacityand/or coverage area of the system. (As described below, thearchitecture allows new VCELLs to be added to the system without acorresponding degradation in performance caused by noise.) Monitoringstations 120 can be added to the LAN 116 as needed to permit the remoteviewing and monitoring of patient data from various locations within thehospital.

(i) General Operation

In operation, the remote telemeters 102 send data packets to individualVCELLs 106 using a wireless TDMA protocol. These packets include thepatient data collected by the remote telemeters 102 (or by patientmonitors connected to the remote telemeters), along with the ID codes ofthe respective telemeters 102. The VCELLs 106 forward these data packetsto the corresponding concentrators 112, which in-turn broadcast thepatient data on the LAN 116 (in real time) for viewing and automatedmonitoring by the monitoring stations 120. The wireless TDMA protocolincludes control timeslots for allowing the VCELLs to pass controlinformation (e.g., synchronization information, commands, and timeslotassignments) to the remote telemeters 102. In addition, the protocolsupports a patient location method (described below) for monitoring theremote location of each patient.

To support patient mobility, the VCELLs 106 and remote telemeters 102implement a "switch-over" protocol in which the telemeters 102continuously attempt to establish connections with those VCELLs whichoffer the best link performance. As part of this protocol, each remotetelemeter continuously assesses the quality of the RF link to each VCELLthat is within range. The telemeters store this link assessmentinformation within respective VCELL "catalogs" (described below), andperiodically evaluate these catalogs to determine whether a switch-overto a new VCELL is desirable. When a remote telemeter 102 determines thata VCELL is available (i.e., has an open timeslot) which offers betterlink performance than a current VCELL (i.e., a VCELL to which thetelemeter is currently connected), the remote telemeter attempts toconnect to the new VCELL. (As described below, this involves sending atimeslot request message to the selected VCELL 106, and then waiting forconfirmation message from the VCELL.) If the connection is successfullyestablished, the remote telemeter 102 drops its connection to thecurrent VCELL 106. Thus, a remote telemeter 102 will normally connect tomany different VCELLs 106 (including VCELLs of different concentrators112) as the patient moves throughout the hospital. Transitions betweenVCELLs occur without interruption or loss of data, and are thus seamlessfrom the viewpoint of the monitoring clinician.

To provide protection against dropouts caused by multi-path interference(and other types of interference), each remote telemeter 102 attempts tomaintain a connection with two VCELLs 106 at all times. (In otherimplementations, the remote telemeters 102 may connect to three or moreVCELLs 106 to provide even greater protection against multi-pathinterference.) Whenever two VCELL connections are established, theremote telemeter 102 transmits each of its data packets to both of theVCELLs. These redundant transfers take place on different frequenciesduring different TDMA timeslots. Thus, each wireless data path benefitsfrom the protection offered by space, time, and frequency diversity.Upon receiving the redundant packets, the concentrator 112 to which thetwo VCELLs 106 are connected (assuming the VCELLs are connected to thesame concentrator) uses error detection codes contained within thepackets to discard bad packets, and to discard duplicate packets whenboth packets are successfully received.

In one implementation of the system, the remote telemeters 102 can onlyconnect to the VCELLs 106 of one concentrator 112 at-a-time. In thisimplementation, each remote telemeter 102 attempts to stay connected tothe VCELLs of the current concentrator 112, and switches over to adifferent concentrator only when deemed necessary. In anotherimplementation, the concentrators 112 of the system are maintainedsufficiently synchronized with one another to allow each remotetelemeter to connect to VCELLs of two different concentrators 112. Whenthis situation occurs, the task of discarding duplicate packetsautomatically shifts to the monitoring stations 120.

The operation of the system is described in further detail in thefollowing sections.

(ii) Hardware Components (FIGS. 3-5A)

FIG. 3 illustrates the basic components of the concentrators 112 andVCELLs 106 of the system. Each concentrator 112 comprises a generic PChaving two RS-422 input-output (I/O) cards 302 and a 100BaseTx LAN card304. The PC may, for example, be a Pentium-based PC with 16 megabytes ofmemory. (Additional memory and a display monitor will normally beprovided if the concentrator 112 is to double as a monitoring station.)The RS-422 and 100BaseTx cards 302, 304 are standard AT size componentswhich can be purchased off-the-shelf at computer stores.

Each RS-422 card 302 includes eight external (full duplex) I/Os whichconnect, respectively, to eight standard twisted pair lines 110. Eachtwisted pair line 110 connects to a respective VCELL 106. The twistedpair lines 110 are preferably shielded 140 Kbaud lines with RJ-45connectors. As is conventional, each twisted pair line includes fourwires: a transmit (TX) wire, a receive (RX) wire, a positive voltage (+)wire, and a negative voltage (-) wire. The (+) and (-) wires are used toprovide power to the VCELLs 106, and the (RX) and (TX) wires are usedfor the transfer of data.

Each VCELL 106 is in the form of a microcontroller-based transceiver 308coupled to an antenna 312. The specifications of a transceiver which maybe used in the preferred embodiment are listed in Table 1. (Atransceiver circuit which may be used within the VCELLs is illustratedin FIG. 5A, and is described below.) The transceiver 308 is coupled torandom access memory (RAM) 310 for buffering packet data and storingvarious status information.

                  TABLE 1                                                         ______________________________________                                        VCELL TRANSCEIVER SPECIFICATIONS                                              ______________________________________                                        Operating Frequency                                                                             204-216 MHz                                                 Frequency Tuning  100 KHz                                                     Transmit Power    1500 μV/meter @ 3 meters                                 Modulation Type   FSK                                                         Modulation Rate   80 Kbaud                                                    Deviation         ±50 KHz                                                  Receive Sensitivity                                                                             -90 dBm (BER < .001)                                        Tx/Rx Switching Time                                                                            <10 μs                                                   Antenna           1/2 Wave Turnstile                                          Power Supply      6 to 12 VDC, <100 ma                                        ______________________________________                                    

As depicted in FIG. 4, each remote telemeter 102A includes conventionalsensor circuitry 402 for sensing and digitizing the patient data of arespective patient. (In instrument remote telemeters 102B of the typeshown in FIG. 1, the sensor circuitry normally resides primarily withinthe patient monitor 104.) The sensor circuitry 402 is coupled to amicrocontroller-based remote transceiver 404, which is in-turn coupledto a RAM 406 and an antenna 408. The sensor circuitry 402, remotetransceiver 404 and RAM 406 are powered by one or more batteries 412.The specifications of a remote transceiver 404 which may be used in thepreferred embodiment are listed in Table 2.

                  TABLE 2                                                         ______________________________________                                        REMOTE TELEMETER                                                              TRANSCEIVER SPECIFICATIONS                                                    ______________________________________                                        Operating Frequency                                                                            204-216 MHz                                                  Frequency Tuning 100 KHz                                                      Transmit Power   1500 μV/meter @ 3 meters                                  Modulation Type  FSK                                                          Deviation        ±50 KHz                                                   Modulation Rate  80 Kbaud                                                     Receive Sensitivity                                                                            -90 dBm (BER < .001)                                         Tx/Rx Switching Time                                                                           <10 μs                                                    Antenna          1/2 Wave Turnstile                                           Power Supply     2 Alkaline Batteries, <25 ma                                 ______________________________________                                    

Although the architecture of FIG. 1 is not tied to any particulartransceiver implementation, the remote transceiver circuit disclosed inthe above-referenced provisional application (diagram reproduced as FIG.5A) is well-suited for use as both the VCELL transceiver 308 and theremote transceiver 404. With reference briefly to FIG. 5A, the circuitincludes a microcontroller 502 (preferably a 17C42) coupled to an EEPROM512 which includes a firmware program stored therein. In the remotetelemeters 102, the firmware program implements the remote telemeterside of the wireless TDMA protocol (described below). Likewise, in theVCELLs 106, the firmware program implements the VCELL side of thewireless TDMA protocol (also described below). An overview of thetransceiver circuit of FIG. 5A is provided below under the headingTRANSCEIVER CIRCUIT AND OPERATION.

(iii) Patient Capacity and Data Throughput

Each VCELL 106 can receive the patient data of six patients (i.e., sixremote telemeters 102) at a sustained maximum data rate of 10 Kbaud perpatient. (This data rate corresponds to one timeslot per TDMA frameusing a simple FM transmitter, as described below.) In addition, thearchitecture supports increased data rates at the expense of reducedpatient capacity. For example, a VCELL 106 could receive the patientdata of three patients (i.e., three remote telemeters 102) at a datarate of 20 Kbaud per patient.

The total patient capacity of the system is limited primarily by thethroughput of the LAN 116. In the preferred embodiment, the systemdesign supports approximately 900 patients at a data rate of 10 Kbaudper patient 102. This results in a backbone throughput requirement of900×10 Kbaud=9 megabaud (Mbaud) at the network level. The use of100BaseTx for the backbone supports this data rate while providing amargin of over 90% for overhead processing (such as synchronization,signaling, and background status keeping tasks). The patient capacity ofthe system can be increased by adding a second backbone 118 to the LAN116 to provide dual 100 Mbaud data paths.

(iv) Noise Floor Improvement (FIG. 6)

One significant benefit of the VCELL architecture is that it overcomesthe above-described noise-floor degradation problem encountered withdistributed antenna systems, and thus allows the system to be expandedin capacity (through the addition of VCELLs) without a correspondingreduction is signal quality. To illustrate the noise-floor degradationproblem encountered with distributed antenna systems, reference will bemade to FIG. 6, which illustrates example dynamic range values for asingle-antenna system (left-hand side) and a 400 antenna system withpreamplifiers (right-hand side) operating within the VHF medicaltelemetry band.

In general, telemetry systems operate between two limits of transmittedsignal strength: (i) the minimum signal that the receiver can detectabove the thermal noise floor (which is the "natural" noise floorcreated by the normal movement of charged particles), and (ii) themaximum signal that the transmitter can provide at very close range (asexperienced when the transmitter resides directly below the receiver'santenna.) As illustrated in FIG. 6, the minimum detectable signal level(based on the thermal noise floor) in the single-antenna telemetrysystem will typically be about -90 dBm. The maximum allowed signal levelwithin the VHF medical telemetry band is 1500 microvolts/meter at 3meters (as specified by FCC Part 15.241), which corresponds to a signallevel of about -40 dBm with the transmitter located directly below aceiling-mounted telemetry antenna. Thus, a single-antenna medicaltelemetry system will have a dynamic range of about 50 dB.

As indicated above, the process of combining the RF signals of theantennas of a distributed antenna system has a loss associated with it.This loss results from the need for signal combiners, and from the largeamount of coaxial cable required to interconnect the various antennas.To compensate for this loss, distributed antenna systems usepreamplifiers, typically at the antenna sites, to boost the RF signal.One problem with this approach is that each preamplifier contributes tothe noise level of the antenna system in excess of the noise actuallyreceived by the corresponding antenna. Thus, although only a few of theantennas typically receive a usable signal of a particular telemeter atany given time, all of the antennas (preamplifiers) contribute to thenoise floor.

Consequently, each time the number of antennas of thedistributed-antenna system is doubled, the minimum detectable signallevel increases by about a factor of 2, or 3 dB. Thus, for example, asystem with 400 antennas and 400 preamplifiers will suffer from a noisefloor degradation of more than 24 dB (corresponding to over 8 doublingsof the noise floor), producing a degraded dynamic range of less than 26dB (FIG. 6). (A distributed antenna system of this size is currentlyinstalled at Barnes Hospital in St. Louis.) To reclaim this lost dynamicrange, the remote telemeters could potentially be operated at a highertransmission power. However, the use of a higher transmission powerwould reduce the average battery life of remote telemeters. Moreover, anincrease in power beyond the limits imposed by FCC Part 15.241 wouldrequire operation outside the protected medical telemetry band, exposingthe system to new forms of RF interference.

In contrast to distributed antenna systems, the VCELL system combinesthe outputs of the VCELLS at baseband using digital multiplexingtechniques. As a result, virtually no degradation of the noise flooroccurs as VCELLs are added to the system, and the system enjoys the full50 dB dynamic range regardless of the number of VCELLs. This has theeffect of increasing the perceived transmitted power by about 24 dB (a200 fold increase) over the 400 antenna system in the example above.

(v) Protection Against Isolated EMI Sources

Another benefit of the VCELL architecture is that it inherently offers ahigh degree of immunity against isolated sources of EMI (electromagneticinterference). In the above-described distributed antenna system, asingle source of interference (such as an X-ray machine or a faultycopying machine) near one of the antennas can introduce an intolerablelevel of noise to the entire system, and prevent the monitoring of allpatients of the system. In contrast, in the VCELL system, theinterference source will only effect the operation of the VCELLs 106that are sufficiently close to the source. Moreover, the contaminationof one or two VCELLs by an isolated interference source will often havelittle or no impact on the ability to monitor patients in the area,since each remote telemeter 102 normally maintains data connections totwo VCELLs 106, and automatically connects to a new VCELL source when adrop in the quality of a VCELL link is detected.

One benefit of this interference immunity is that it allows patients tobe monitored near known intermittent sources of interference. Forexample, patients can be monitored within x-ray, fluoroscopy, andCAT-scan rooms by simply placing VCELLs in these areas.

(vi) VCELL Spacing and Frequency Reuse (FIG. 7)

The VCELLs are preferably mounted sufficiently close to one another suchthat each patient of the system will normally be within range ofmultiple VCELLs 106 at any given time. (For operation at maximum powerwithin the VHF medical telemetry band, spacings of 50 to 75 feet aresuitable.) Such an arrangement allows the remote telemeters 102 tomaintain connections with two VCELLs at-a-time, as is desirable formitigating the effects of multi-path interference.

One benefit of the architecture, however, is that it allows the VCELLsto be spaced as closely together as necessary to accommodate differentpatient densities. For example, a relatively large number of VCELLs(each operating on a different frequency) can be placed within ahospital cafeteria to accommodate the high patient densities which mayoccur during meal times. Because the remote telemeters 102 only attemptto connect to the VCELLs 106 that have open timeslots (as describedbelow), the telemetry load during such high-density events isautomatically distributed among the VCELLs.

Although it is possible to configure the system such that every VCELL106 operates (i.e., transmits and receives data) on its own uniquefrequency, considerable performance benefits (described below) can berealized by re-using the same set of frequencies in different regions ofthe hospital. In general, two VCELLs can operate on the same frequencyprovided that they are sufficiently spaced apart to avoid interferencewith one another. For operation within the VHF medical telemetry band(at the maximum allowed signal strength), a separation of 500 feetbetween such VCELLS is more than adequate. By assigning like frequenciesduring the installation process to VCELLS that are spaced 500 feet (orgreater) apart, it is estimated that 10 to 12 frequencies will besufficient to provide coverage for a typical hospital.

FIG. 7 illustrates how a set of ten frequencies can be re-used indifferent sections of a hospital hall 700. As illustrated, a set of tenfrequencies, θ1-θ10, can be used to cover a 500 foot section of the hallusing ten corresponding VCELLs. (Each frequency symbol in FIG. 7represents one VCELL.) The same ten frequencies can then be used tocover the next 500 foot section of the hallway. With appropriatestaggering of frequencies between hospital floors, the same 10frequencies can be used to provide coverage of an entire multi-floorhospital. (While ten frequencies may be adequate for many installations,the actual number of frequencies will depend upon such factors as thehospital floor plan, the telemeter transmission power, and the expectedpatient densities in the various patient areas.)

The ability to reuse frequencies provides several advantages overconventional frequency division multiplexed systems. One advantage isthat a reduced number of clear frequencies need to be identified duringthe installation process. Another advantage is that the transmitters,receivers and antennas of the system can be optimized to operate over amuch narrower band of frequencies. For a system which operates withinthe VHF medical telemetry band, for example, the VCELL frequencies canbe selected to fall within a band of one or two VHF television channels(such as channels 12 and/or 13, which tend to have the lowest ambientnoise), rather than spanning the entire 174 to 216 MHz range. It isestimated that such optimization will add a performance margin of 6 to10 dB over existing telemetry equipment which operates over the entire174 to 216 MHz range.

2. Communications Between Remote Telemeters and VCELLS (FIGS. 8-12 )

(i) Overall Wireless TDMA Protocol (FIG. 8)

FIG. 8 illustrates a single frame of the wireless TDMA protocol usedbetween the remote telemeters 102 and the VCELLs 106. The frame repeatsevery 5 milliseconds, and consists of seven timeslots: a 740 microsecond(μs) VC→R (VCELL to remote telemeter) timeslot and six 710 μs R→VC(remote telemeter to VCELL) timeslots. The VC→R timeslots are used tobroadcast information to the remote telemeters 102. (As described below,all VCELLs of the system are synchronized, and thus transmit at the sametime.) The R→VC timeslots are assigned by the VCELLs 106 to individualtelemeters 102, and are used to transfer information from the assignedtelemeters to the VCELLs. All timeslots terminate with a 10 μs deadperiod, which is sufficient to allow the devices to switch betweentransmit and receive modes.

In the preferred embodiment, the remote telemeters 102 and VCELLs 106transmit at a raw data rate of 80 Kbaud, which corresponds to a bit timeof 12.5 μs. At this data rate, a total of (700 μs/slot)/(12.5 μs/bit)=56bits are transmitted during the data portion of each R→VC timeslot. Thefirst six of the 56 bit times are used for synchronization of thereceiver, leaving 50 bits for the transfer of telemetry data (includingerror detection codes). Because this 50 bit message repeats every 5milliseconds, or 200 times per second, the total telemeter-to-VCELLthroughput for a single timeslot assignment is 200×50=10,000bits/second, or 10 Kbaud. This throughput rate is obtained in thepreferred embodiment using simple FM transceivers in the VCELLs andtelemeters. As will be recognized by those skilled in the art, higherthroughput rates can be achieved, at a greater expense, by usingtransceivers which use more sophisticated modulation techniques, such asBPSK and QPSK.

With further reference to FIG. 8, the VCELLs broadcast respectivecontrol messages to the remote telemeters 102 during the first 700 μs ofeach VC→R timeslot. (Because all VCELLs within range of one anothertransmit on different frequencies, each telemeter 102 can listen to thecontrol message of only one VCELL at-a-time.) The control messages areused to transmit the following information to the telemeters:

Synchronization Sequences.

The telemeters use these sequences to initially become synchronized andto maintain synchronization with the VCELLs 106.

VCELL-Specific Timeslot Assignment Status Data.

For a given VCELL, this status data indicates which of the six R→VCtimeslots (if any) are unassigned, and thus available for use. Thetelemeters use this information to formulate timeslot request messagesto the VCELLs.

Telemeter-Specific Timeslot Assignment Messages.

A timeslot assignment message (or "confirmation" message) is transmittedin response to a timeslot request message from a specific telemeter, andserves as an acknowledgement to the telemeter that it has successfullyacquired the requested timeslot.

Telemeter-Specific Commands.

This is an optional feature which may be supported by certain remotetelemeters 102. A command may be sent, for example, to instruct atelemeter to take a blood pressure reading, or to enter into specialmode of operation.

VCELL ID Codes.

Each VCELL transmits a unique ID code which is used for patientlocation.

During the last 30 μs of each VC→R timesiot, each VCELL transmits alow-power (1/4-power in the preferred embodiment), unmodulated signal toallow each remote telemeter 102 to estimate the location of therespective patient. (The use of a low-power, unmodulated signal for thispurpose produces a more accurate VCELL-telemeter distance measurement.)Each remote telemeter 102 measures the signal strengths of the low-powertransmissions of the various VCELLs (by listening to different VCELLfrequencies during different TDMA frames), and maintains a table(discussed below) of the detected signal strengths. As a low duty cycletask (e.g., once every 5 seconds), each telemeter 102 evaluates itsrespective table to estimate the closest VCELL (i.e., the VCELL with thegreatest signal strength). Whenever a change occurs in the closestVCELL, the telemeter transmits the ID of the new VCELL to the hospitalLAN 116 (FIG. 1). The monitoring stations 120 use this information tokeep track of the locations of the patients of the system. In otherembodiments of the invention, patient location may be accomplished byhaving the VCELLs periodically attach VCELL identification codes to thedata packets received from the remote telemeters 102.

With further reference to FIG. 8, the six R→VC timeslots are used by theremote telemeters 102 to transmit data packets to individual VCELLs 106.(As described below, each telemeter 102 transmits to only one VCELLat-a-time.) These data packets are generally of two types: (i) telemetrydata packets which include the patient data (including patient locationdata) of individual patients, and (ii) timeslot request messages forrequesting timeslot assignments. Once a R→VC timeslot has been assignedby a VCELL to a remote telemeter, the remote telemeter has exclusive useof the timeslot until the telemeter disconnects. Once the telemeterdisconnects, the VCELL modifies its control message (transmitted on theVC→R timeslot) to indicate that the timeslot is available for use.

During normal operation, each R→VC timeslot of a given VCELL 106 will beassigned, if at all, to a different remote telemeter 102. Thus, when allsix R→VC timeslots of the VCELL are assigned, the VCELL receives thetelemetry data of six different remote telemeters 102. In other modes ofoperation, multiple timeslots of a single VCELL can be assigned to thesame telemeter to allow the telemeter to achieve a higher datathroughput rate.

(ii) VCELL Protocol (FIGS. 9 and 10)

The VCELL side of the above-described wireless TDMA protocol isimplemented via a firmware program which is executed by themicrocontroller of each VCELL 106. With reference to FIG. 9, thisprogram maintains a timeslot status table 900 in VCELL RAM 310 to keeptrack of the assignment status of each R→VC timeslot. As illustrated inFIG. 9, the information stored within the table 900 includes, for eachtimeslot, whether or not the timeslot is currently assigned orunassigned. In addition, for the timeslots that are assigned, the tableindicates the number of consecutive frames that have passed withoutreceiving an error-free data packet from the assigned telemeter; eachVCELL uses this information to implement a timeout procedure todetermine whether the assigned telemeter 102 has disconnected.

FIG. 10 is a flow chart which illustrates the VCELL portion of thewireless protocol. With reference to block 1002, during a power-oninitialization sequence, the VCELL 106 updates its timeslot status table900 to set all of the R→VC timeslots to the "free" (unassigned) state.The VCELL then enters into a primary program loop which corresponds to asingle TDMA frame. Referring to blocks 1004 and 1006 of this loop,during the VC→R timeslot the VCELL transmits the 710 μs control messagefollowed by the 30 μs patient location signal, as illustrated in FIG. 8.This control message includes the timeslot assignment data (for all sixR→VC slots) stored in the timeslot status table 900. On the first passthrough this loop following power-on, the control message will indicatethat all six R→VC timeslots are available for use. With reference toblock 1008, the VCELL 106 then sets a slot counter (N) to one(corresponding to the first R→VC timeslot), and enters into a sub-loop(blocks 1012-1032) for processing the data packets transmitted by thetelemeters.

During each R→VC timeslot, the VCELL attempts to receive any telemeterdata packet transmitted during the timeslot (block 1012), and checks thetimeslot status table 900 to determine whether the slot is assigned(block 1014). With reference to blocks 1016-1022, if the timeslot isassigned and a data packet was successfully received, the VCELL forwardsthe packet to the concentrator 112, and clears the corresponding "missedpackets" counter in the status table 900. (The data packet is actuallysent to the concentrator 112 following the TDMA frame as part of alarger "VCELL packet" which represents the entire frame.) If, on theother hand, the timeslot is assigned but no packet was successfullyreceived, the VCELL 106 updates the timeslot status table 900 toindicate that a packet was missed; in addition, the VCELL determineswhether the number of consecutive missed packets has exceeded a timeoutthreshold (e.g., 64 packets). If the threshold is exceeded, the statustable 900 is updated to set the timeslot to the "free" state.

With reference to blocks 1026 and 1028, if the timeslot is unassigned,the VCELL 106 determines whether it received a valid timeslot requestmessage. If a valid timeslot request was received, the VCELL updates itsstatus table to indicate that the slot has been assigned; in addition,the VCELL sets a flag to indicate that a timeslot confirmation messageshould be transmitted to the requesting telemeter 102 during the nextVC→R timeslot.

With reference to blocks 1030 and 1032, once all processing of thereceived telemeter packet (if any) is performed, the VCELL incrementsits timeslot counter. If the incremented counter is 6 or less, theprogram loops back to block 1012 to begin receiving any data transmittedduring the next timeslot. If the incremented counter has exceeded 6, theprogram loops back to block 1004 to transmit the next control message.

Although the FIG. 10 flowchart illustrates the above-describedoperations in sequential order, it will be recognized that some of theseoperations can (and normally will) be performed concurrently or out-oforder. For example, the step of forwarding the data packets to theconcentrator (block 1018) is preferably performed as a separate task,with all of the telemeter packets received during the TDMA frametransferred together within a larger VCELL packet.

(iii) Remote Telemeter Protocol (FIGS. 11 and 12)

The telemeter side of the wireless TDMA protocol generally mirrors theVCELL protocol 1000, and is similarly implemented by a firmware programwhich is executed by the microcontroller of each remote telemeter 102.As part of this protocol, each remote telemeter 102 maintains a VCELLcatalog within its respective RAM 406. The general format of thiscatalog is illustrated in FIG. 11, which is representative of a systemwhich uses a total of 10 VCELL frequencies. As illustrated in FIG. 11,the catalog 1100 includes one set of entries for each of the ten VCELLfrequencies. The telemeter thus stores status information for up to tennearby VCELLs at-a-time. The entries stored with respect to each VCELLfrequency include the following:

Rating.

A rating of the quality of the RF link to the VCELL 106, as assessed bythe individual telemeter 102. The RF links are assessed by thetelemeters one frequency at-a-time during the control message portionsof the VC→R timeslots. (In other embodiments, the task of assessing theavailable RF links may alternatively be performed by the VCELLs.) In oneembodiment, the VCELL ratings are based on a combination of signalstrength (as measured by the telemeters) and bit error rate.Specifically, if an error is detected in a VCELL's transmission, theVCELL's rating is set to a zero or null entry to indicate that the VCELLshould not be used; and if no error is detected, the rating is set to avalue which is proportional to the measured signal strength. The ratingsare periodically compared (as described below) to determine whether toattempt a connection to a new VCELL.

Connected To.

A flag indicating whether or not the telemeter is connected to a VCELLon the frequency. During normal operation, each telemeter will beconnected to two different VCELLs (on two different frequencies)at-a-time.

Low-Power Signal Strength.

A measurement of the signal strength taken during the low-power (patientlocation) portion of the VC→R timeslot. The low-power signal strengthsstored in the catalog 1100 are periodically compared to estimate whichof the VCELLs the patient is closest to. When the outcome of thiscomparison changes, the telemeter transmits the new location (i.e., theVCELL ID, which is obtained from the VCELL during the high-power messageportion) to the hospital LAN 116 in a subsequent data packet.

The remote telemeters 102 also keep track of the unique IDs of theVCELLs that are within range.

The protocol followed by the remote telemeters 102 generally consists ofthe following steps:

1. Scan all VCELL operating frequencies and construct the VCELL catalog1100. Remain in this mode until at least one VCELL 106 is identifiedwhich has an acceptable rating and an unassigned timeslot.

2. Send a timeslot request message to the VCELL identified in step 1during one of the available timeslots. Perform this task using a randomback-off algorithm in case other remote telemeters attempt to connect tothe VCELL during the same timeslot. Remain in this operating mode(including step 1) until a timeslot assignment message is received fromthe selected VCELL.

3. Once connected to a first VCELL ("VCELL 1"), attempt to connect tothe "next best" VCELL ("VCELL 2") in the catalog which has an acceptablerating and a free timeslot (other than the timeslot being used tocommunicate with VCELL1), to provide a second (diversity) data path.Send telemetry data packets to VCELL1 during this process.

4. Monitor the catalog entries to determine whether any of the otherVCELLs offer better link performance than VCELLs 1 and 2. When a betterVCELL is available (i.e., has an open, nonconflicting timeslot), send atimeslot request message to the "new" VCELL. Drop the connection withthe current VCELL once a timeslot assignment message is received.

5. As a background task, scan the VCELL frequencies and update thecatalog. This can be done as a low priority, low duty cycle task (suchas once per second).

FIG. 12 illustrates this protocol in greater detail for a system whichuses 10 VCELL frequencies. With reference to block 1202, the remotetelemeter 102 initially scans the ten VCELL frequencies and builds theVCELL catalog 1100. This involves monitoring different VCELL frequenciesduring different TDMA frames. With reference to blocks 1204-1210, oncean acceptable VCELL 106 with an open timeslot has been identified, thetelemeter 102 transmits a timeslot request message to the selectedVCELL, and then monitors the selected VCELL's frequency during thefollowing VC→R timeslot to determine whether the slot has beensuccessfully acquired. With reference to block 1214, if the connectionattempt is unsuccessful, a random back-off algorithm (such as the binaryexponential back-off algorithm used by Ethernet) is used to retry theconnection attempt. If the retry is unsuccessful (after, for example, 2retry attempts), or if it is determined that the requested timeslot hasbeen assigned to a different telemeter 102, the telemeter repeats theabove process to identify another potential VCELL.

With reference to blocks 1220-1226, once a timeslot assignment has beenobtained from a first VCELL, the protocol enters into a loop (blocks1222-1240) which corresponds to a single TDMA frame. Within this loop,the telemeter 102 sends telemetry data packets to the first VCELL (block1222) during the assigned timeslot while attempting to connect to asecond VCELL (block 1226). The process of connecting to the second VCELLis generally the same as the above-described process for connecting tothe first VCELL, with the exception that the timeslot used tocommunicate with the second VCELL must be different from the timeslotused to communicate with the first VCELL. With reference to block 1228,once a second VCELL connection has been established, the telemeter sendsall data packets to both VCELLs.

With reference to blocks 1232 and 1234, the remote telemeter 102monitors the high-power and low-power transmissions of the VCELLS 106(during the VC→R timeslots) and updates the rating and low-power signalstrength entries of the VCELL catalog 1100. Although this process isshown in FIG. 12 as occurring during every TDMA frame (one VCELL perframe), this function can alternatively be performed as a low duty cycletask.

With reference to blocks 1238 and 1240, as a background task thetelemeter 102 monitors the VCELL catalog 1100 to determine whether aVCELL 106 with a higher rating exists. If a VCELL with a higher ratingand an available (nonconflicting) timeslot is identified, the telemeterattempts to connect to the new VCELL (as described above), and ifsuccessful, drops the existing connection to one of the two "current"VCELLs.

As a background, low priority task, the telemeter 102 also monitors theVCELL catalog 1100 to determine whether a change has occurred in theVCELL 106 with the greatest low-power signal strength. (This process isomitted from FIG. 12 to simplify the drawing.) When such a changeoccurs, the telemeter transmits the VCELL ID of the new "closest" VCELLin a subsequent telemetry packet. As described above, this informationis used by the monitoring stations 120 to track the location of eachpatient.

3. Communications Between VCELLs and Concentrators (FIG. 13)

The VCELLs 106 and concentrators 112 communicate bi-directionally overthe shielded twisted pair lines 110 in accordance with the RS-422specification. (RS-422 is an Electronic Industries Association interfacestandard which defines the physical, electronic and functionalcharacteristics of an interface line which connects a computer tocommunications equipment.) The RS-422 interface supports an overall datatransfer rate of 140 Kbaud, which corresponds to 20 Kbaud per TDMAtimeslot.

In operation, each VCELL 106 sends one packet to its respectiveconcentrator 112 for every TDMA frame; this VCELL packet includes all ofthe telemeter packets (up to six) received during the corresponding TDMAframe. (The terms "VCELL packet" and "telemeter packet" are used in thisdescription to distinguish between the two types of packets based ontheir respective sources.) The concentrator 112 in-turn parses the VCELLpacket to extract the individual telemeter packets, and performs errorchecking on the telemeter packets using the error detection codescontained within such packets. As part of the error checking protocol,the concentrator 112 discards all telemeter packets which include errors(or uncorrectable errors if error correction codes are used), anddiscards all telemeter packets that are redundant of packets alreadyreceived from a different VCELL. All other packets are written to anoutput buffer for subsequent broadcasting over the LAN 116.

FIG. 13 is a flow chart which illustrates the concentrator side of theVCELL-to-concentrator protocol in further detail. With reference toblock 1302, the concentrator 112 sends a VCELL synchronization pulse toits 16 VCELLs once per TDMA frame. The concentrator then initializes aloop counter (block 1304), and enters into a loop (blocks 1306-1324) inwhich the concentrator processes the 16 VCELL packets (one per loop)received from the 16 VCELLs. Within this loop, the concentrator 112parses each VCELL packet (block 1306) to extract the individualtelemeter packets contained therein, and then enters into a sub-loop(blocks 1310-1320) in which the concentrator performs error checking (asdescribed above) on the individual telemeter packets (one telemeterpacket per sub-loop).

With reference to blocks 1310-1316, error free telemeter packets whichhave not already been successfully received (from other VCELLs) arewritten to an output buffer of the concentrator 112. (As describedbelow, a separate concentrator task reads these packets from the bufferand broadcasts the packets on the LAN 116 during patient-specifictimeslots of the LAN protocol.) With reference to blocks 1320-1324, onceall of the telemeter packets within a given VCELL packet have beenprocessed, the protocol loops back to block 1306 (unless all 16 VCELLpackets have been processed, in which case a new synchronization pulseis transmitted), and the concentrator 112 begins to process the nextVCELL packet.

As indicated by the foregoing, the concentrators 112 only placeerror-free telemeter packets on the LAN backbone 118, and do not placeduplicate error-free telemeter packets (from the same telemeter) on thebackbone 118. Nevertheless, the duplicate (error-free) packetstransmitted by a telemeter will normally appear on the LAN backbone 118when the remote telemeter connects to VCELLs of two differentconcentrators 112 (as permitted in one implementation of the invention).In this situation, the monitoring stations 120 simply ignore the extratelemeter packets.

(i) Processing of Telemeter Commands

In system implementations which support the sending of commands to theremote telemeters 102, the concentrators 112 additionally implement asimple task (not illustrated in FIG. 13) for receiving commands from themonitoring stations 120 and forwarding these commands to the VCELLs 106.As part of this task, each concentrator 112 maintains a list of all ofthe remote telemeters 102 to which the concentrator is currentlyconnected. (This list is generated by monitoring the telemeter ID codescontained within the telemeter data packets.) When a monitoring station120 places a telemeter-addressed command on the LAN 116, eachconcentrator 112 of the system receives the command and checks itsrespective list to determine whether a connection exists with the targettelemeter. If a concentrator determines that such a connection currentlyexists, the concentrator 112 sends the command to all 16 of its VCELLs106. The sixteen VCELLs in-turn transmit the telemeter command during asubsequent VC→R timeslot. To increase the probability of receipt, theVCELLs are preferably configured to re-transmit the telemeter commandover several TDMA frames. In other embodiments, an acknowledgementprotocol can be implemented in which the telemeters embed anacknowledgement message within a subsequent data packet.

4. Data Transfers Over LAN

Data transfers over the LAN backbone 118 are accomplished using areal-time TDM (time division multiplexing) protocol which makes use ofthe 100BaseTx protocol. Among other things, this protocol distributesthe telemetry data from the concentrators 112 to the monitoring stations120 with a known latency, permitting the real-time monitoring of patientdata.

Each 50 millisecond frame of the TDM protocol includes 1000, 50 μstimeslots. Every remote telemeter 102, VCELL 106, concentrator 112,monitoring station 120, and gateway 124 of the system is uniquelyassigned one of the 1000 backbone timeslots. The backbone timeslots thatare uniquely assigned to respective remote telemeters 102 are used totransfer telemetry data packets (containing patient-specific physiologicdata) from the concentrators 112 to the monitoring stations 112. Allother entities that are connected to the LAN backbone 118 also haveaccess to this telemetry data. The remaining backbone timeslots are usedfor the transfer of synchronization and control information between thevarious LAN entities.

The 100BaseTx backbone 118 has the capacity to transfer up to 5000 bitsin each 50 μs backbone timeslot. Thus, each remote telemeter (and otherentity which is assigned a backbone timeslot) is effectively allocated aLAN bandwidth of 5000 bits/slot×20 frames/second=100 Kbaud. This morethan satisfies the 20 Kbaud data rate per telemeter which is requiredwhen a telemeter connects to VCELLs of two different concentrators.

Upon initialization of the system, a "master" concentrator 112 transmitsa synchronization packet to all other concentrators of the LAN 116. Thissynchronization packet defines the starting point of the backbone TDMframe. Thereafter, the frame repeats at a rate of 20 frames per second.As indicated above, a task which runs on each concentrator movestelemeter packets from the concentrator's output buffer to the LANduring the appropriate patient-specific (50 μs) timeslots. This taskwaits for a patient (telemeter) timeslot, and then transmits allcorresponding telemeter packets which have been written to the outputbuffer since the same timeslot of the immediately preceding backboneframe. When a telemeter is connected to VCELLs of two differentconcentrators, the 50 μs patient timeslot is divided equally between thetwo concentrators. This is accomplished by passing control messagesbetween the concentrators.

5. VCELL Load Monitoring

As a background task, each concentrator 112 maintains a statistical logor "histogram" of the loads carried by each of the concentrator's VCELLs106. This histogram can periodically be examined by networkadministrators to evaluate the current positioning of the VCELLs. When,for example, the histogram indicates that a VCELL in a particularpatient area reaches its capacity (i.e., all six timeslots assigned) ona frequent basis, another VCELL (which operates on a differentfrequency) can be installed in the area to reduce the load on theheavily-loaded VCELL.

6. Transceiver Circuit and Operation (FIGS. 5A and 5B) The transceivercircuit illustrated in FIG. 5A will now be described. As indicatedabove, this general circuit can be used in both the remote telemeters102 and the VCELLs 106 of the system. When included within a remotetelemeter 102, the transceiver will typically be powered by battery.When included within a VCELL 106, the transceiver will be powered by thecorresponding concentrator 112 over a twisted pair line 110 (asillustrated in FIG. 3).

The transceiver 112 comprises a microcontroller (preferably a 17C42)which is connected, via appropriate port lines, to a programmablephase-locked loop chip 504 ("PLL chip"), a voltage controlled oscillator(VCO) 506, a receiver (RCVR) 508, a set of DIP (dual in-line package)switches 510, an EEPROM 512, and a sample-and-hold (S/H) device 520.(The sample-and-hold 520 is preferably omitted in the VCELL transceivers308.) The PLL chip 504 is preferably a Motorola MC 145192 which can beplaced, via appropriate commands, into a low-power state when not inuse. The microcontroller 502 is clocked by an 8 MHz high stability(±0.001 %) crystal oscillator 516. The output of the amplifier 524 andthe signal input of the receiver 508 are connected to respectiveterminals of a transmit/receive switch 528, which is connected to theantenna 312, 408 via a band-pass filter (BPF) 530.

As illustrated in FIG. 5A, the PLL chip 504 is coupled to the VCO 506 toform a phase lock loop circuit. Via the PLL chip 504, the phase lockloop circuit can be programmed to generate a carrier signal of aselected frequency. Within the transceivers of the telemeters 102, thesample-and-hold device 520 is connected so as to allow themicrocontroller 502 to programmably interrupt the phase-lock process andhold the carrier frequency at a steady frequency value within apreselected margin of frequency error. This allows the carrier frequencyto be locked rapidly, at low power, without waiting for a phased-lockedstate to be reached. In a preferred embodiment, this feature is used tolock the transmit frequency of each telemeter just prior to each R→VCtimeslot to which the telemeter is assigned.

As illustrated in FIG. 5A, the VCO 506, amplifier 524, and receiver 508are coupled to the power supply (V_(SUP)) via respectivemicrocontroller-controlled switches 534, 536, 538 such that themicrocontroller 502 can selectively turn these components ON and OFF toconserve power. (In the VCELLs, the switches 534, 536, 538 can beomitted since battery life is not a concern.) To utilize thispower-conservation feature, the firmware program (stored within theEEPROM 512) of the remote telemeters 102 includes code for maintainingthese active transceiver components in an OFF state when not in use. Forexample, the receiver is maintained in an OFF state during TDMAtimeslots for which the remote telemeter 102 is not receiving data, andthe amplifier is maintained in an OFF state during timeslots for whichthe remote telemeter 102 is not transmitting. This feature of thetransceiver circuit significantly increases the average battery life ofthe remote telemeters 102.

In operation within a remote telemeter, the microcontroller 502maintains the PLL chip 504 in its low-power state, and maintains theamplifier 424, VCO 506 and receiver 508 in respective OFF states, duringtimeslots for which the telemeter is neither transmitting nor receivingdata. Shortly before the next R→VC timeslot which is assigned to thetelemeter 102, the microcontroller 502 initiates a frequency lockoperation which involves initiating a phase-lock process, and theninterrupting the process (by opening the sample-and-hold 520) once thecarrier frequency has settled to within an acceptable margin of error.This process is illustrated in Figure SB, which is an approximate graphof the output (V_(PLL)) of the PLL chip 504 following power-up at T₀.

With reference to FIG. 5B, just prior to T₀, the VCO 506 is turned on,the sample-and-hold 520 is in the closed (or "sample") position, and thePLL chip 504 is in the low-power state. At To, the PLL chip 504 is takenout of the low-power state, causing its output V_(PLL) to ring, and thuscausing the output of the VCO to oscillate above and below theprogrammed transmit frequency. Following T₀, the output of the PLL is inthe general form of a damped sinusoid, which approaches the voltage thatcorresponds to the programmed frequency. (Because the voltage V_(PLL)controls the VCO 506, the amplitude of the voltage signal in FIG. 5Bcorresponds to the frequency.) Once this oscillation is sufficientlyattenuated such that the frequency error is within a predeterminedtolerance (e.g., ±5 KHz), the sample-and-hold 520 is opened (at T₁ inFIG. 5) to hold the input voltage to the VCO 506. (This is accomplishedby waiting a predetermined delay, T_(DELAY), before opening thesample-and hold 520, as described in the above-referenced priorityapplication.) This holds the output frequency, and ensures that theremote telemeter's subsequent data transmission will not be contaminatedby any oscillation in the PLL's output. Immediately following T1, theamplifier 524 is turned on, the PLL 504 is placed in the low-powerstate, and the T/R switch 528 is placed in the transmit position. Themicrocontroller 402 then begins sending its transmit data to the VCO, tothereby FSK-modulate the carrier signal. Following the transmission ofthe telemeter packet, the amplifier 524, and VCO 506 are turned off.

In one embodiment, the telemeter firmware is written such that thetelemeters 102 only use non-adjacent (i.e., non-consecutive) R→VCtimeslots. This ensures that each telemeter will have at least a 720 μs"dead period" between transmissions during which to lock the newtransmit frequency using the above-described process.

The process of receiving data (during VC→R timeslots) is generallyanalogous to the above-described transmit process, with the exceptionthat the sample-and-hold 520 is left in the closed (sample) positionthroughout the VC→R timeslot.

7. Conclusion

While the present invention has been described herein with reference toa preferred embodiment of a medical telemetry system, the invention isnot so limited, and should be defined only in accordance with thefollowing claims.

What is claimed is:
 1. A medical telemetry system for permittingreal-time monitoring of patients, including ambulatory patients, of amedical facility from a centralized monitoring station, comprising:atleast one monitoring station which displays real-time physiologic dataof multiple patients; a plurality of battery-powered wireless remotetelemeters, the remote telemeters configured to attach to, andconfigured to collect and transmit real-time physiologic data of,respective patients; and a plurality of transceivers which communicatebi-directionally with the plurality of remote telemeters using aplurality of wireless radio frequency (RF) channels, the transceiversconnected to the at least one monitoring station and being distributedthroughout the medical facility such that different transceivers providedata-reception coverage for different areas of the medical facility, atleast some of the transceivers receiving physiologic data on differentRF channels, the transceivers configured to receive the physiologic datatransmitted by the remote telemeters and to forward the physiologic dataover a wired connection to the at least one monitoring station;whereinat least some of the remote telemeters automatically switch betweendifferent RF channels of the plurality of RF channels in response topatient movement throughout the medical facility to connect to anddisconnect from individual transceivers of the plurality oftransceivers.
 2. The medical telemetry system according to claim 1,wherein at least some of the remote telemeters evaluate wireless linkconditions offered by different transceivers and establish wirelessconnections with individual transceivers based at least upon saidwireless link conditions.
 3. The medical telemetry system according toclaim 2, wherein at least one remote telemeter of the plurality ofremote telemeters maintains wireless connections with at least twodifferent transceivers at-a-time, and transmits the physiologic data ofa respective patient to the at least two different transceivers toprovide at least spacial diversity.
 4. The medical telemetry systemaccording to claim 2, wherein the remote telemeters communicate with thetransceivers using a TDMA protocol, and the at least one remotetelemeter transmits the physiologic data to the at least two differenttransceivers during different respective TDMA timeslots to therebyadditionally provide time diversity.
 5. The medical telemetry systemaccording to claim 4, wherein the at least one remote telemetertransmits the physiologic data to the at least two differenttransceivers on different RF frequencies, the different RF frequenciesspaced sufficiently apart to provide frequency diversity.
 6. The medicaltelemetry system according to claim 1, wherein the transceivers forwardthe physiologic data to the at least one monitoring station over a localarea network.
 7. The medical telemetry system according to claim 1,wherein at least two of the transceivers communicate with the remotetelemeters on the same RF channel.
 8. The medical telemetry systemaccording to claim 7, wherein the at least two transceivers whichcommunicate on the same RF channel are sufficiently spaced apart toavoid interference with one another.
 9. The medical telemetry systemaccording to claim 1, wherein at least some of the transceivers of theplurality of transceivers are synchronized with one another.
 10. Themedical telemetry system according to claim 1, wherein different subsetsof the plurality of transceivers are connected to respectiveconcentrator computers, each concentrator computer receiving physiologicdata from a respective subset of transceivers and forwarding thephysiologic data to the at least one monitoring station.
 11. The medicaltelemetry system according to claim 10, wherein the concentratorcomputers are interconnected by a local area network.
 12. The medicaltelemetry system according to claim 11, wherein the local area networkcomprises a plurality of monitoring stations which display the real-timephysiologic data of the patients.
 13. The medical telemetry systemaccording to claim 1, wherein the remote telemeters communicate with thetransceivers using a TDMA protocol, and at least one of the transceiversbroadcasts timeslot availability messages to the remote telemeters, thetimeslot availability messages indicating available and unavailable TDMAtimeslots for communicating with the at least one transceiver.
 14. Themedical telemetry system according to claim 13, wherein differenttransceivers of the plurality broadcast timeslot availability messagesto the remote telemeters on different RF frequencies.
 15. The medicaltelemetry system according to claim 1, wherein the plurality oftransceivers communicate with the plurality of remote telemeters withina VHF medical telemetry band.
 16. The medical telemetry system accordingto claim 1, wherein at least one of the remote transceivers maintainswireless connections with multiple remote telemeters at-a-time.
 17. Themedical telemetry system according to claim 1, wherein at least one ofthe remote transceivers is positioned proximate to a known intermittentsource of electromagnetic interference.
 18. The medical telemetry systemaccording to claim 17, wherein the at least one remote transceiver ispositioned within a patient X-ray room.
 19. The medical telemetry systemaccording to claim 1, wherein at least some of the transceivers transmitpatient location signals to the remote telemeters.
 20. The medicaltelemetry system according to claim 1, wherein at least one remotetelemeter of the plurality of remote telemeters includes a processorwhich executes a control program to implement a wireless communicationsprotocol in which the remote telemeter transmits data packets todifferent transceivers during different timeslots, the control programconfigured to maintain wireless connections with at least two differenttransceivers at-a-time on at least two different RF channels, and totransmit data packets that contain like physiologic data to the at leasttwo different transceivers to provide multiple simultaneous transmissionpaths for the transfer of the physiologic data between the remotetelemeter and the monitoring station.
 21. A method of transferringreal-time physiologic data of a patient from a wireless remote telemeterto a receiving node of a network so as to provide protection againstmulti-path interference, the remote telemeter configured to attach tothe patient, the method comprising:providing first and secondtransceivers that are positioned within respective first and secondpatient areas of a medical facility, the patient areas beingsufficiently close to one another so that the transceivers provideoverlapping first and second coverage zones, each transceiver positionedremotely from and being connected to the receiving node; transmitting afirst data packet from the remote telemeter to the first transceiverduring a first timeslot, the first data packet containing at least thereal-time physiologic data; transmitting a second data packet from theremote telemeter to the second transceiver during a second timeslotwhich is different from the first timeslot, the second data packetcontaining at least the real-time physiologic data; forwarding the firstand second data packets, respectively, from the first and secondtransceivers to the receiving node; and selecting between the first andsecond data packets at the receiving node.
 22. The method according toclaim 21, wherein the first and second data packets contain respectiveerror detection codes, and the step of selecting comprises evaluatingthe error detection codes at the receiving node to determine whether thefirst and second data packets were successfully received by the firstand second transceivers.
 23. The method according to claim 22, whereinthe error detection codes comprise error correction codes.
 24. Themethod according to claim 21, wherein the step of transmitting the firstdata packet is performed on a first frequency channel and the step oftransmitting the second data packet is performed on a second frequencychannel, the first and second frequency channels sufficiently differentto provide frequency diversity.
 25. The method according to claim 24,wherein the first and second frequency channels fall within a VHFmedical telemetry band.
 26. The method according to claim 21, furthercomprising sending either the first data packet or the second datapacket from the receiving node to a patient monitoring station over awired local area network.
 27. The method according to claim 21, whereinthe step of forwarding the first and second data packets to thereceiving node comprises transmitting the packets over differentrespective wired connections.
 28. The method according to claim 21,wherein the steps of transmitting first and second packets from theremote telemeter are repeated periodically to transmit the patient's ECGwaveform data in real-time to the receiving node.
 29. The methodaccording to claim 21, further comprising:positioning a thirdtransceiver within a third patient area to provide a third coverage zonewhich overlaps with the first and second coverage zones, the thirdtransceiver positioned remotely from and being connected to thereceiving node; and in response to patient movement away from the firstpatient area towards the third patient area, automatically establishinga connection between the remote telemeter and the third transceiver andterminating a connection between the remote telemeter and the firsttransceiver.
 30. The method according to claim 29, wherein the step ofestablishing a connection with the third transceiver comprises sending atimeslot request message from the remote telemeter to the thirdtransceiver.
 31. The method according to claim 29, wherein the step ofestablishing a connection with the third transceivercomprises:broadcasting a timeslot availability message from the thirdtransceiver, the timeslot availability message indicating available andunavailable timeslots for communicating with the third transceiver; andreceiving the timeslot availability message with the remote telemeter,and evaluating the timeslot availability message to identify anavailable timeslot.
 32. The method according to claim 29, wherein thefirst, second and third transceivers communicate with the remotetelemeter on different respective frequency channels.
 33. A medicaltelemetry system for monitoring patients of a medical facility, thesystem comprising:a plurality of transceivers distributed throughout themedical facility such that different transceivers provide data receptioncoverage for different areas of the medical facility, the plurality oftransceivers connected to a common data receiving node, at least some ofthe transceivers of the plurality operating on different radio frequency(RF) channels; and a wireless remote telemeter that is configured to beworn by an ambulatory patient, the remote telemeter including sensingcircuitry for sensing physiologic data of the patient, the remotetelemeter implementing a wireless communications protocol in which theremote telemeter transmits data packets to different transceivers of theplurality during different timeslots, and in which the remote telemetermaintains wireless connections with at least two different transceiversat-a-time on at least two different RF channels and transmits likephysiologic data to the at least two different transceivers to providemultiple concurrent transmission paths for the transfer of thephysiologic data from the remote telemeter to the receiving node. 34.The medical telemetry system according to claim 33, wherein the remotetelemeter implements a switch-over protocol in which the remotetelemeter automatically connects to and disconnects from differenttransceivers of the plurality based on assessments of RF linkconditions.
 35. The medical telemetry system according to claim 33,wherein the physiologic data comprises ECG waveform data, and the remotetelemeter transmits the ECG waveform data to the at least two differenttransceivers in real-time.
 36. The medical telemetry system according toclaim 32, wherein each transceiver is configured to concurrently receivethe physiologic data of multiple patients from multiple telemeters.